Seattle Mariners LHP Justus Sheffield Does it Different

The Mariners 2020 season is set to be an interesting one. Young players will fill out the roster in an attempt to see what they have to offer the team as they look to open their window of contention in the next few years.

Among the youngsters, LHP Justus Sheffield, acquired by the Mariners in the 2018 James Paxton trade, is likely to open the 2020 campaign as a member of the rotation.

Sheffield owned a 5.50 ERA over the course of 36 IP. He struck out 37 while walking 18. Of course, those numbers don’t tell the full story. In the case of Sheffield, they hardly tell the story at all. He began the year in AAA Tacoma and, just like every other pitcher in AAA, got pummeled.

He did make one appearance in the big leagues, filling in as the headliner after Yusei Kikuchi pitched in one of his planned abbreviated starts. Before too long, Sheffield found himself back down in AA Arkansas with Pete Woodworth, who at the time was the Travelers pitching coach and has since been promoted to the same role with the big league club.

Sheffield returned to the big leagues in August and looked like he had figured it out. In September, the lefty faced the Cubs in Chicago and threw the best game of his young career. He struck out 7 over five innings, didn’t give up a run and walked only 2. He finished the season with a rather high.371 BABIP while also sporting a 52.3 GB%.

The telling number here is the Ground Ball Percentage. Clearly, the 23-year-old is a ground ball pitcher and the numbers seem to back that up. Sheffield has a low spin rate on his fastball, averaging out to 1835 RPM at an average of 92.5 MPH according to Baseball Savant. That’s good for a Bauer Unit measure of 19.8 which is well below league average for a fastball.

His fastball also drops an average of 22 inches whereas the league average is 17. Because of this, Sheffield is better suited to pitch down in the zone and the numbers would suggest, based on his increased GB%, that Sheffield has done just that. Of course, because of this, Sheffield will likely never be a high strikeout pitcher but outs are outs.

League average for fastball spin rate is right around 2200 so it’s easy to see why Sheffield could find success with such a slow spin rate. Hitters are used to facing an average spin rate so the further away you can get from that number, the better off you are going to be.

Sheffield was ranked 515th out of 517 players In 2019 for average fastball spin rate. Interestingly enough, the Mariners have two players in the top ten. Carl Edwards Jr. (7th) and Austin Adams (10th) both rank amongst the top fastball spin rates in the game of baseball.

Sheffield pairs his fastball with a slider, which is typical of a pitcher who throws from a 3/4 arm slot. He had a .302 BAA on his slider, which hitters swung at 50% of the time and of those swings, missed 47.5% of the time.

He threw his slider roughly 36% of the time resulting in 29 of his 37 strikeouts. Clearly, Sheffield uses his slider as an out pitch. His slider doesn’t move much from the 0,0 point, suggesting he puts a decent amount of gyro spin on the pitch.

During the game against the Cubs, Sheffield threw 40 sliders and only one of them was put in to play whereas 9 of the 42 fastballs he threw ended up in play. Over the course of the season, he had a .636 BABIP on his slider

His three-pitch mix is complemented by a changeup, which is his worst offering. He threw 110 of them over the course of the season, giving up eight hits and boasted wOBA of .336. For Sheffield to reach his full potential, he will likely need to improve his pitch design for his changeup or develop a new pitch altogether.

Realistically, it wouldn’t hurt Sheffield to work on tunneling his pitches better. He releases he fastball from about 5’5 above the rubber whereas his changeup is released from 5’4. It’s not a huge difference, but a small change could have a drastic impact on the effectiveness of Sheffield.

It’s pretty clear to see how Sheffield stands out amongst the rest of the league. His 5’10 frame already comes out on the smaller end of the spectrum for pitches. The data seems to suggest that Sheffield’s strength will be living at the bottom of the zone and throwing his slider while he’s ahead in counts.

It will be interesting to see, once he gets a full body of work, how Sheffield’s pitches play at the big leagues. A full season should give the Mariners a better idea about the lefties’ future, whether it be in the rotation or out of the bullpen.

2020 is going to be another rough year for the M’s. Every team in the division has gotten better, nobody can argue that. But it will be an important step for each of the next young core to show what they can do. Sheffield’s season may be more important than most. He represents a huge part of the Mariners’ future rotation and we are getting closer to see exactly what kind of pitcher he can be.